<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=, initial-scale=1.0">
    <title>选择三个盒子</title>
    <style>
        .box {
            width: 100px;
            height: 100px;
            font-size: 30px;
        }

        .red  {
            background-color: red;
        }

        .green {
            background-color:green;
        }
    </style>
</head>
<body>
    <div class="box red">红色</div>
    <div class="box green">绿色</div>
    <div class="box  red">红色</div>
</body>
</html>